June 30, 2014
The high-brightness 1-micron cutting laser is redefining productivity in metal fabrication. The laser’s good absorption helps it cut faster with less power. It also means that keeping optics clean and dust-free is essential.
Say you need to replace your laser’s cover slide, the sacrificial lens that has become ubiquitous on most high-power 1-micron solid-state laser cutting heads. You remove the slide, and then your cell phone rings, so you head off to another area to take the call. Fifteen minutes later you return and install the fresh cover slide so that the laser can resume operation. You download the program, initiate the cutting cycle, and find your edge quality isn’t up to standards. What happened?
The laser work envelope isn’t always the cleanest of environments. When you left to take the call, minute particles floated up into the laser head optic. According to several sources from laser cutting head manufacturers, in the 1-micron laser world, especially when using higher laser powers, a small amount of dust can make a big difference in cut quality and optics life.
In recent years, those in the sheet metal cutting arena has embraced the1-micron solid-state laser, and for good reason. Cutting throughput on thin stock can be jaw-dropping, and material handling automation increases potential throughput even more. The systems are straightforward. You have a delivery fiber that emerges from the laser power source to connect the laser cutting head—no mirrors, no laser gas, and the collimator that shapes the beam resides in the cutting head itself. It’s an extremely efficient way to transform light into an extraordinarily useful state (see Figures 1 and 2).
Even highly reflective material readily absorbs the beam’s 1-micron wavelength. Because the beam has such a high energy density and absorbs so well, it also can cut quickly and with less laser power. In effect, it can do more with less. But these same characteristics also mean that a very small particle caught in its path can cause problems.
Still, with some of the latest cutting optics and heads, fabricators shouldn’t have a need to access the inside of the cutting head often, other than to swap out a cover slide. If an operation does require a lens changes in-house, it’s done in a clean area separate from the manufacturing environment. Other operations may require sending the head to an outside vendor to have the optics cleaned or replaced.
Some 1-micron laser cutting heads have focusing optics that can be accessed, while other heads are sealed completely, save for the cover slide. “We are moving more in the direction of removing the possibility of having the user-accessible lens on the cutting head,” said Robert Borgstrom, president of Precitec Inc., Wixom, Mich. “The cutting head is preconfigured for a certain optical ratio.”
As cutting head manufacturers explained, cleaning or replacing optics does require technicians to take extra precautions, but that’s a small price to pay for the 1-micron laser’s cutting efficacy. The best practice is to follow maintenance recommendations spelled out by machine and cutting head vendors, so specific inspection and cleaning procedures won’t be covered here. But no matter what procedure fabricators follow, a common theme pervades: Best practices aim to prevent contaminants from getting into the processing head in the first place.
Years from now we’ll probably look back at today as a time of great change in the laser cutting arena. Most expect the 1-micron market to take off in the coming years, and proprietary technologies are making machines more flexible than ever. But regardless of the specifics behind all these proprietary technologies, everyone approaches laser optics on the same photonics playing field.
“There are three principles as to how light interacts with materials. Material will either absorb the light, reflect the light, or transmit the light.” So said Mike DelBusso, senior sales engineer at Novi, Mich.-based Laser Mechanisms. “All materials have a combination of these properties. When it comes to laser optics, you want to get the maximum transmission through the focusing lens with as little reflectivity as possible. When the laser light reaches the material, you want to have as much absorption of this energy and very little reflectivity.”
Most lenses for 1-micron lasers are made of fused silica, which transmits the 1-micron wavelength extremely well, as opposed to zinc selenide, which works best with the CO22 laser’s 10.6-micron wavelength. Although fused silica works best with the 1-micron wavelength, it doesn’t conduct heat as well as zinc selenide.
“Zinc selenide [on a CO2 laser] can take heat away from debris [on the lens] that’s absorbing laser energy,” said Tom Kugler, fiber systems manager at Laser Mechanisms. “In a 1-micron system, smaller particles will cause absorption. And when lenses start to absorb energy, they distort how they focus. It’s called a thermal focus shift, or thermal lensing. So cleanliness is very important.”
“Absorption is basically the transformation of light into heat,” said Detlev Wolff, head of sales for HIGHYAG, a subsidiary of Saxonburg, Pa.-based II-VI Inc. “Excessive heat in the optics will damage something. You need to avoid contamination at all times. This is key when moving from 10.6-micron to 1-micron lasers. The physics are basically the same, but the technology is quite different.”
Wolff added that besides cleanliness, the alignment between the optical elements is critical. “So in some operations, it has turned out not to be practical [to swap out the focusing lens], though there have always been users who do it well. Ultimately, it has been all about how flexible you can be with your existing optics. The solution has been to use motorized optics.”
Fiber laser cutting heads are designed with cleanliness in mind. Behind the nozzle is usually a sacrificial cover slide, which protects the focusing lens above it. Above that is the collimator, which shapes the light that has traveled through a delivery fiber from the laser source.
Modern processing heads can adjust focus position and diameter automatically by moving optical elements.
As sources explained, two optical elements—the focusing lens and collimator—work together to create the spot diameter and position. This allows the system to cut different material thicknesses without manual intervention (see Figures 3 and 4).
Which particular head a shop uses depends on the cutting machine it buys, and various proprietary approaches exist. A cutting head may not need every bell and whistle. Even basic processing heads may be able to cut multiple thicknesses, depending on application requirements. Regardless of the type of cutting head, one trend is clear: In many operations, you probably won’t need to change the focusing optic.
In most laser cutting applications, the beam itself technically doesn’t cut material but simply melts it. The assist gas really acts as the “cutting tool,” evacuating molten metal to create the kerf. The pressure must be carefully controlled, especially during the pierce. Before the laser penetrates completely, there’s no place for the metal to go but up.
The pressure and volume of the assist gas are critical. Too low and the pierce won’t penetrate effectively, too high and excess spatter will stick to the nozzle and the cover slide. A hole-intensive or small-part nest can have a lot of pierces. Frequent cover slide changes may be a sign that cutting parameters, specifically the assist gas flow, need refinement. Today’s laser machines have proportional valves controlled by the CNC, which draws from a table of cutting parameters that include material type and thickness, for both piercing and cutting portions of the operation.
Certain materials produce a dirtier cutting environment than others; galvanized or similar material in which the process releases microscopic particles from the sheet’s zinc coating is one example. “This results in a very fine dust,” Borgstrom said. “It can get everywhere.” He added, however, that as long as you swap cover slides correctly and refrain from otherwise exposing the optics unnecessarily, modern cutting heads should remain clean and dust-free.
With the machine idle and area clear of smoke and debris, a technician can swap out the cutting head’s cover slide. Removing the cover slide may expose the lens to the atmosphere, and the longer that lens is exposed, the greater chance it has to become contaminated (see Figure 5). “This is where we suggest having two cover slide cartridges, including one that is pre-mounted with a clean cover slide,” said Borgstrom. “So when you remove the old one, you slip in the new one right after that. And the fresh cover slide should be sealed on both surfaces. The fresh slide should never be left on a bench somewhere.”
Heads also can have integrated predictive maintenance systems. “You have an optical sensor detecting the contamination,” Wolff said. “If you have contamination on the cover slide, this changes your optical properties, and this in turn changes your contamination. Ideally, you want to see contamination before there is a change in optical quality. So there are sensors there to detect what the status of the cover slide really is.”
Some also have another cover slide positioned above the collimator to protect optics if for whatever reason the cutting head needs to be removed. If a head is removed, that top cover slide can be replaced.
Processing heads now are being designed with internal purge gas to prevent dust and debris from reaching the optics. Some setups have cover slides between the cutting head and delivery fiber, which adds another layer of protection.
Setups vary, depending on cost requirements and the specific laser machine a shop has. Regardless of what system a machine has, sources recommended that if you need to remove the cutting head, make sure the delivery fiber end cap is secure. The end cap should be stored in a clean place, not on top of a workbench or anywhere else it could be exposed to shop dust. Otherwise, small particles from the end cap can fall into the delivery fiber itself. Also, when possible you should disconnect the head in the horizontal position. This prevents dust from floating down into the laser head.
Everyone emphasized that it’s important to put these precautions in perspective. The optics for high-powered 1-micron lasers are sensitive, but the situation is certainly manageable. It’s a small trade-off, considering the 1-micron laser’s throughput potential and minimal overall maintenance requirements. The laser itself is solid-state with no moving components.
Like any manufacturing consumable, laser optics wear naturally over time. But according to sources, most optics problems occur in two areas of the processing head: the top and bottom. Debris on the bottom lens surface usually is a sign that the optics may have had prolonged exposure to a dirty environment during a cover-slide change. Debris on the top lens in the processing head is a sign that particles may have floated down into the head after the fiber was disconnected.
In other words, most major problems occur not from the laser cutting process itself or even from natural wear, but from improper handling. Ultimately it’s all about minimizing exposure. As sources explained, this is best practice for any laser, be it of the CO2 or 1-micron variety. If the cut edge is clean, the optics probably are as well. If done in a dirty environment, opening a cutting head to inspect what’s inside may well do more harm than good.
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